This invention relates to a system and method for creating features at the bottom of a cavity. Embodiments of this invention relate to a method for making an infrared transmitting lid for an infrared-emitting or infrared detecting device using sub-wavelength structures.
Microelectromechanical systems (MEMS) are very small, often moveable structures made on a substrate using surface or bulk lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
MEMS may also be non-moving devices, such as photonic devices, fabricated using surface or bulk lithographic processing techniques. In such cases, small features required for the device to emit radiation in a narrow spectrum, for example, may be formed using MEMS techniques. Such a photonic device is a photonic crystal, formed from two metal films separated by a dielectric, with small holes formed in the metal films which determine the radiation output pattern and spectrum of the device. The device may be heated to an operating temperature of about 350 degrees centigrade, by driving a current through the device, and heating it by Joule heating. When the operating temperature is achieved, the photonic device may emit the desired spectrum of radiation, often in the infrared portion of the electromagnetic spectrum.
Because the device may emit radiation in the infrared portion of the spectrum, for example, at a wavelength of between about 8 um and about 12 um, it is often desirable to encapsulate the photonic device in a vacuum cavity of a lid wafer that transmits the infrared radiation. Providing the vacuum cavity may reduce the absorption losses of the radiation that might otherwise occur if a gaseous environment, such as air, surrounds the infrared device. However, in order to also reduce reflective losses from the surfaces of the vacuum cavity, it may be desirable to provide antireflective structures, such as thin optical coatings on the cavity surfaces. One option for providing such a low reflectivity vacuum cavity is to deposit an antireflective coating on the lid wafer with a lift off method. However, the lift off method may require a thick photoresist, which is hard to remove after the antireflective coating is deposited on the lid wafer, because of the very high temperatures which must be used to deposit the antireflective coating.
Another option for providing the infrared-transmitting lid is to pattern sub-wavelength structures on one or both surfaces of the lid wafer, as described in, for example, U.S. Pat. No. 6,897,469, incorporated by reference in its entirety. The subwavelength features effectively reduce the dielectric constant of the lid wafer material and thereby reduce its reflection coefficient, as described in the incorporated '469 patent. However, in order to make the subwavelength features within the device cavity of the lid wafer, a relatively deep cavity may first be formed, sufficient to clear the infrared device, and the sub-wavelength features may be formed at the bottom of this cavity. The high resolution lithography required to form these small features is difficult to accomplish when the features are located at the bottom of a cavity, because the lithography cannot be focused easily on the surface on which the features are to be formed. In addition it is very difficult to obtain a uniformly thick layer of photosensitive material when using standard coating methods in the presence of such extreme topography, and significant variations in thickness of the photosensitive material can make patterning of small features very difficult or impossible.